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      通过源码详解 Servlet
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        <h2 id="Servlet-结构"><a href="#Servlet-结构" class="headerlink" title="Servlet 结构"></a>Servlet 结构</h2><p><img src="http://image.54tianzhisheng.cn/servlet1.png" alt="servlet1"></p>
<h3 id="1、Servlet"><a href="#1、Servlet" class="headerlink" title="1、Servlet"></a>1、Servlet</h3><p>Servlet 该接口定义了5个方法。<br><a id="more"></a></p>
<ul>
<li>init()，初始化 servlet 对象，完成一些初始化工作。它是由 servlet 容器控制的，该方法只能被调用一次</li>
<li>service()，接受客户端请求对象，执行业务操作，利用响应对象响应客户端请求。</li>
<li>destroy()，当容器监测到一个servlet从服务中被移除时，容器调用该方法，释放资源，该方法只能被调用一次。</li>
<li>getServletConfig()，ServletConfig 是容器向 servlet 传递参数的载体。</li>
<li>getServletInfo()，获取 servlet 相关信息。</li>
</ul>
<p>Servlet 的生命周期：</p>
<blockquote>
<p>1，初始化阶段  调用 init() 方法</p>
<p>2，响应客户请求阶段　　调用 service() 方法</p>
<p>3，终止阶段　　调用 destroy() 方法</p>
</blockquote>
<p>在 Servlet 接口中的五个方法中涉及的接口有三个：ServletConfig 、 ServletRequest、 ServletResponse</p>
<p>这里先讲讲 ServletRequest 和 ServletResponse。</p>
<p>1）ServletRequest  由 Servlet 容器来管理，当客户请求到来时，容器创建一个 ServletRequest 对象，封装请求数据，同时创建一个 ServletResponse 对象，封装响应数据。这两个对象将被容器作为 service（）方法的参数传递给 Servlet，Serlvet 利用 ServletRequest 对象获取客户端发来的请求数据，利用 ServletResponse 对象发送响应数据。</p>
<p>下面是 ServletRequest  中所有的方法，根据方法名大概就可以猜到这些方法到底是干啥用的。</p>
<p><img src="http://image.54tianzhisheng.cn/ServletRequest.png" alt="ServletRequest"></p>
<p>2）ServletResponse 发送响应数据</p>
<p><img src="http://image.54tianzhisheng.cn/ServletResponse.png" alt="ServletResponse"></p>
<h3 id="2、ServletConfig"><a href="#2、ServletConfig" class="headerlink" title="2、ServletConfig"></a>2、ServletConfig</h3><p>ServletConfig 是容器向 servlet 传递参数的载体。</p>
<p>ServletConfig的4个常用方法：</p>
<p>1）public String getInitParameter（String name）：返回指定名称的初始化参数值；</p>
<p>2）public Enumeration getInitParameterNames（）：返回一个包含所有初始化参数名的 Enumeration 对象；</p>
<p>3）public String getServletName()：返回在 DD 文件中<code>&lt;servlet-name&gt;</code>元素指定的 Servlet 名称；</p>
<p>4）public ServletContext getServletContext（）：返回该 Servlet 所在的上下文对象；</p>
<p>这里详细讲下 ServletContext ：</p>
<p><img src="http://image.54tianzhisheng.cn/ServletContext.png" alt="ServletContext"></p>
<p>Servlet 上下文对象（ServletContext）：每个Web应用程序在被启动时都会创建一个唯一的上下文对象，Servlet 可通过其获得 Web 应用程序的初始化参数或 Servlet 容器的版本等信息，也可被 Servlet 用来与其他 Servlet 共享数据。</p>
<p>1、获得 ServletContext 应用：</p>
<p>（1）、直接调用 getServletContext（）方法</p>
<p>ServletContext context = getServletContext（）;</p>
<p>（2）、使用 ServletConfig 应用，再调用它的 getServletContext（）方法</p>
<p>ServletContext context = getServletConfig.getServletContext();</p>
<p>2、获得应用程序的初始化参数：</p>
<p>（1）、public String getInitParameter（String name）：返回指定参数名的字符串参数值，没有则返回 null；</p>
<p>（2）、public Enumeration getInitParameterNames()：返回一个包含多有初始化参数名的 Enumeration 对象；</p>
<p>3、通过 ServletContext 对象获得资源</p>
<p>（1）、public URl getResource（String path）:返回由给定路径的资源的 URL 对象，以 “/” 开头，为相对路径，相对于Web 应用程序的文档根目录；</p>
<p>（2）、public InputStream getResourceAsStream（String path）：从资源上获得一个 InputStream 对象，等价于getResource（path）.oprenStream();</p>
<p>（3）、public String getRealPath(String path)：返回给定的虚拟路径的真实路径；</p>
<p>4、登陆日志：使用 log（）方法可以将指定的消息写到服务器的日志文件中</p>
<p>（1）、public void log（String msg）：参数 msg 为写入日志文件消息</p>
<p>（2）、public void log（String msg，Throwable throwable）：将 msg 指定的消息和异常的栈跟踪信息写入日志文件</p>
<p>5、使用 RequestDispatcher 实现请求转发</p>
<p>（1）、RequestDispatcher getRequestDiapatcher(String path)：必须以 “/“ 开头相对于应用程序根目录，而ServletRequest 可以传递一个相对路径</p>
<p>（2）、RequestDipatcher getNamedDiapatcher（String name）：参数 name 为一个命名的 Servlet 对象</p>
<p>6、使用 ServletContext 对象存储数据</p>
<p>（1）、public void serAttribute（String name，Object object）：将给定名称的属性值对象绑定到上下文对象上；</p>
<p>（2）、public Object getAttribute（String name）：返回绑定到上下文对象的给定名称的属性值；</p>
<p>（3）、public Enumeration getAttributeNames()：返回绑定到上下文对象上的所有属性名的 Enumeration 对象；</p>
<p>（4）、public void removeAttribute（String name）：删除绑定到上下文对象指定名称的属性；</p>
<p>ServletRequest 共享的对象仅在请求的生存周期中可以被访问；</p>
<p>HttpSession 共享的对象仅在会话的生存周期中可以被访问；</p>
<p>ServletContext 共享的对象在整个 Web 应用程序启动的生存周期中可以被访问；</p>
<p>7、检索 Servlet 容器的信息</p>
<p>（1）、public String getServletInfo()：返回 Servlet 所运行容器的名称和版本；</p>
<p>（2）、public int getMajorVersion（）：返回容器所支持的 Servlet API 的主版本号；</p>
<p>（3）、public int  getMinorVersion（）：返回容器所支持的 Servlet API 的次版本号；</p>
<p>（4）、public String getServletContext（）：返回 ServletContext 对应的 web 应用程序名称 <code>&lt;display-name&gt;</code>元素定义的名称；</p>
<h3 id="3、GenericServlet-抽象类"><a href="#3、GenericServlet-抽象类" class="headerlink" title="3、GenericServlet 抽象类"></a>3、GenericServlet 抽象类</h3><p>GenericServlet  定义了一个通用的，不依赖具体协议的 Servlet，它实现了 Servlet 接口和 ServletConfig 接口，它的方法在文章的第一张图就给出了。</p>
<h3 id="4、HttpServlet-抽象类"><a href="#4、HttpServlet-抽象类" class="headerlink" title="4、HttpServlet 抽象类"></a>4、HttpServlet 抽象类</h3><h4 id="4-1、HTTP-请求方式"><a href="#4-1、HTTP-请求方式" class="headerlink" title="4.1、HTTP 请求方式"></a>4.1、HTTP 请求方式</h4><p><img src="http://image.54tianzhisheng.cn/http-request-method.jpg" alt="http-request-method"></p>
<ol>
<li>GET : 获取由请求 URL 标识的资源</li>
<li>POST : 向 Web 服务器发送无限制长度的数据</li>
<li>PUT : 存储一个资源到请求的 URL</li>
<li>DELETE : 删除由 URL 标识的资源</li>
<li>HEAD : 返回 URL 标识的头信息</li>
<li>OPTIONS : 返回服务器支持的 HTTP 方法</li>
<li>TRACE : 返回 TRACE 请求附带的头字段</li>
</ol>
<h4 id="4-2、对应的服务方法："><a href="#4-2、对应的服务方法：" class="headerlink" title="4.2、对应的服务方法："></a>4.2、对应的服务方法：</h4><ol>
<li>doGet() : 调用服务器的资源, 并将其作为响应返回给客户端.  doGet() 调用在 URL 里显示正在传送给 Servlet 的数据,这在系统的安全方面可能带来一些问题, 比如说, 用户登录时,  表单里的用户名和密码需要发送到服务器端, doGet() 调用会在浏览器的 URL 里显示用户名和密码.</li>
<li>doPost() : 它用于把客户端的数据传给服务端, 使用它可以以隐藏方式给服务器端发送数据. Post 适合发送大量数据.</li>
<li>doPut() : 调用和 doPost() 相似, 并且它允许客户端把真正的文件存放在服务器上, 而不仅仅是传送数据.</li>
<li>doDelete() : 它允许客户端删除服务器端的文件或者 Web 页面．它的使用非常少．</li>
<li>doHead() : 它用于处理客户端的 Head 调用,并且返回一个 response. 当客户端只需要响应的 Header 时,它就发出一个Header 请求.这种情况下客户端往往关心响应的长度和响应的 MIME 类型.</li>
<li>doOptions(): 它用于处理客户端的 Options 调用,通过这个调用, 客户端可以获得此 Servlet 支持的方法.如果 Servlet 覆盖了 doPost() 方法, 那么将返回: Allow: POST, TRACE, OPTIONS, HEAD</li>
<li>doTrace：处理 TRACE 请求</li>
</ol>
<h4 id="4-3、Servlet-Service-方法详解"><a href="#4-3、Servlet-Service-方法详解" class="headerlink" title="4.3、Servlet   Service 方法详解"></a>4.3、Servlet   Service 方法详解</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">service</span><span class="params">(ServletRequest req, ServletResponse res)</span></span></span><br><span class="line"><span class="function">        <span class="keyword">throws</span> ServletException, IOException</span></span><br><span class="line"><span class="function">    </span>&#123;</span><br><span class="line">        HttpServletRequest  request;</span><br><span class="line">        HttpServletResponse response;</span><br><span class="line">        <span class="comment">// 如果传入的 HTTP 请求和 HTTP 响应不是 HTTP 的领域模型，则抛出 Servlet 异常，这个异常会被 Servlet 容器所处理</span></span><br><span class="line">        <span class="keyword">if</span> (!(req <span class="keyword">instanceof</span> HttpServletRequest &amp;&amp;</span><br><span class="line">                res <span class="keyword">instanceof</span> HttpServletResponse)) &#123;</span><br><span class="line">            <span class="keyword">throw</span> <span class="keyword">new</span> ServletException(<span class="string">"non-HTTP request or response"</span>);</span><br><span class="line">        &#125;</span><br><span class="line">		<span class="comment">// 既然是 HTTP 协议绑定的 Serlvet, 强制转换到 HTTP 的领域模型</span></span><br><span class="line">        request = (HttpServletRequest) req;</span><br><span class="line">        response = (HttpServletResponse) res;</span><br><span class="line">		<span class="comment">// 如果传入的请求和响应是预期的 HTTP 请求和 HTTP 响应，则调用 HttpServlet 的 service() 方法。</span></span><br><span class="line">        service(request, response);</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>
<h4 id="4-4、HttpServlet-service-方法详解"><a href="#4-4、HttpServlet-service-方法详解" class="headerlink" title="4.4、HttpServlet   service  方法详解"></a>4.4、HttpServlet   service  方法详解</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">service</span><span class="params">(HttpServletRequest req, HttpServletResponse resp)</span></span></span><br><span class="line"><span class="function">        <span class="keyword">throws</span> ServletException, IOException</span></span><br><span class="line"><span class="function">    </span>&#123;</span><br><span class="line">    <span class="comment">// 从 HTTP 请求中取得这次请求所使用的 HTTT 方法</span></span><br><span class="line">        String method = req.getMethod();</span><br><span class="line">		<span class="comment">// 如果这次请求使用 GET 方法</span></span><br><span class="line">        <span class="keyword">if</span> (method.equals(METHOD_GET)) &#123;</span><br><span class="line">        	<span class="comment">// 取得这个 Servlet 的最后修改的时间</span></span><br><span class="line">            <span class="keyword">long</span> lastModified = getLastModified(req);</span><br><span class="line">            <span class="keyword">if</span> (lastModified == -<span class="number">1</span>) &#123;</span><br><span class="line">                <span class="comment">// servlet doesn't support if-modified-since, no reason</span></span><br><span class="line">                <span class="comment">// to go through further expensive logic</span></span><br><span class="line">                <span class="comment">//-1 代表这个 Servlet 不支持最后修改操作，直接调用 doGet() 进行处理 HTTP GET 请求</span></span><br><span class="line">                doGet(req, resp);</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">            	<span class="comment">// 如果这个 Servlet 支持最后修改操作，取得请求头中包含的请求的最后修改时间</span></span><br><span class="line">                <span class="keyword">long</span> ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);</span><br><span class="line">                <span class="keyword">if</span> (ifModifiedSince &lt; lastModified) &#123;</span><br><span class="line">                    <span class="comment">// If the servlet mod time is later, call doGet()</span></span><br><span class="line">                    <span class="comment">// Round down to the nearest second for a proper compare</span></span><br><span class="line">                    <span class="comment">// A ifModifiedSince of -1 will always be less</span></span><br><span class="line">                    <span class="comment">// 如果请求头中包含的修改时间早于这个 Servlet 的最后修改时间，说明这个 Servlet 自从客户上一次 HTTP 请求已经被修改了 , 设置最新修改时间到响应头中</span></span><br><span class="line">                    maybeSetLastModified(resp, lastModified);</span><br><span class="line">                    <span class="comment">// 调用 doGet 进行进行处理 HTTP GET 请求</span></span><br><span class="line">                    doGet(req, resp);</span><br><span class="line">                &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                <span class="comment">// 如果请求头中包含修改时间晚于这个 Servlet 的最后修改时间，说明这个 Servlet 自从请求的最后修改时间后没有更改过，这种情况下，仅仅返回一个 HTTP 响应状态 SC_NOT_MODIFIED</span></span><br><span class="line">                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);</span><br><span class="line">                &#125;</span><br><span class="line">            &#125;</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_HEAD)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 HEAD 方法</span></span><br><span class="line">         <span class="comment">// 如果这个 Servlet 支持最后修改操作，则设置这个 Servlet 的最后修改时间到响应头中</span></span><br><span class="line">            <span class="keyword">long</span> lastModified = getLastModified(req);</span><br><span class="line">            maybeSetLastModified(resp, lastModified);</span><br><span class="line">            <span class="comment">// 和对 HTTP GET 方法处理不同的是，无论请求头中的修改时间是不是早于这个 Sevlet 的最后修改时间，都会发 HEAD 响应给客户，因为 HTTP HEAD 响应是用来查询 Servlet 头信息的操作</span></span><br><span class="line">            doHead(req, resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_POST)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 POST 方法</span></span><br><span class="line">            doPost(req, resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_PUT)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 PUT 方法</span></span><br><span class="line">            doPut(req, resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_DELETE)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 DELETE 方法</span></span><br><span class="line">            doDelete(req, resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_OPTIONS)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 OPTIONS 方法</span></span><br><span class="line">            doOptions(req,resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_TRACE)) &#123;</span><br><span class="line">        <span class="comment">// 如果这次请求使用 TRACE 方法</span></span><br><span class="line">            doTrace(req,resp);</span><br><span class="line"></span><br><span class="line">        &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">            <span class="comment">// Note that this means NO servlet supports whatever</span></span><br><span class="line">            <span class="comment">// method was requested, anywhere on this server.</span></span><br><span class="line">			<span class="comment">// 如果这次请求是其他未知方法，返回错误代码 SC_NOT_IMPLEMENTED 给 HTTP 响应，并且显示一个错误消息，说明这个操作是没有实现的</span></span><br><span class="line">            String errMsg = lStrings.getString(<span class="string">"http.method_not_implemented"</span>);</span><br><span class="line">            Object[] errArgs = <span class="keyword">new</span> Object[<span class="number">1</span>];</span><br><span class="line">            errArgs[<span class="number">0</span>] = method;</span><br><span class="line">            errMsg = MessageFormat.format(errMsg, errArgs);</span><br><span class="line"></span><br><span class="line">            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>
<h3 id="5、Servlet-的多线程问题"><a href="#5、Servlet-的多线程问题" class="headerlink" title="5、Servlet 的多线程问题"></a>5、Servlet 的多线程问题</h3><p>1、当涉及到 Servlet 需要共享资源是，需保证 Servlet 是线程安全的</p>
<p>2、注意事项：</p>
<p>（1）、用方法的局部变量保持请求中的专有数据；</p>
<p>（2）、只用 Servlet 的成员变量来存放那些不会改变的数据；</p>
<p>（3）、对可能被请求修改的成员变量同步（用 Synchronized 关键字修饰）；</p>
<p>（4）、如果 Servlet 访问外部资源，那么需要同步访问这些资源；</p>
<p>3、实现 SingleThreadModel 接口的 Servlet 在被多个客户请求时一个时刻只能有一个线程运行，不推荐使用。</p>
<p>4、如果必须在 servlet 使用同步代码，应尽量在最小的范围上（代码块）进行同步，同步代码越少，Servlet 执行才能越好，避免对 doGet() 或 doPost() 方法同步。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>全文首先通过一张 Servlet 中的核心 Servlet 类图关系，了解了几种 Servlet 之间的关系及其内部方法。然后在分别介绍这几种 Servlet，通过分析部分重要方法的源码来了解，还介绍了 Servlet 中多线程的问题的解决方法。</p>
<p>注：文章原创，首发于：<a href="http://www.54tianzhisheng.cn">zhisheng 的博客</a>，文章可转载但请注明地址为：<a href="http://www.54tianzhisheng.cn/2017/07/09/servlet/">http://www.54tianzhisheng.cn/2017/07/09/servlet/</a></p>
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        <p><span>本文标题:</span><a href="/2017/07/09/servlet/">通过源码详解 Servlet</a></p>
        <p><span>文章作者:</span><a href="/" title="访问 zhisheng 的个人博客">zhisheng</a></p>
        <p><span>发布时间:</span>2017年07月09日 - 00时00分</p>
        <p><span>最后更新:</span>2018年11月22日 - 22时51分</p>
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            <span>原始链接:</span><a class="post-url" href="/2017/07/09/servlet/" title="通过源码详解 Servlet">http://www.54tianzhisheng.cn/2017/07/09/servlet/</a>
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